Process for growing a dielectric layer on a silicon-containing surface using a mixture of N2O and O3

ABSTRACT

This invention is embodied in an improved process for growing high-quality silicon dioxide layers on silicon by subjecting it to a gaseous mixture of nitrous oxide (N 2 O) and ozone (O 3 ). The presence of O 3  in the oxidizing ambiance greatly enhances the oxidation rate compared to an ambiance in which N 2 O is the only oxidizing agent. In addition to enhancing the oxidation rate of silicon, it is hypothesized that the presence of O 3  interferes with the growth of a thin silicon oxynitride layer near the interface of the silicon dioxide layer and the unreacted silicon surface which makes oxidation in the presence of N 2 O alone virtually self-limiting. The presence of O 3  in the oxidizing ambiance does not impair oxide reliability, as is the case when silicon is oxidized with N 2 O in the presence of a strong, fluorine-containing oxidizing agent such as NF 3  or SF 6 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/059,718,filed Apr. 13, 1998, now U.S. Pat. No. 6,607,946, issued Aug. 19, 2003,which is a continuation of application Ser. No. 08/651,563, filed on May22, 1996, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to integrated circuit manufacturing technologyand, more specifically, to processes for growing insulative layers byconsumption of an underlying layer.

2. State of the Art

In the manufacturing of integrated circuits, the growth of high-qualityinsulative layers for device isolation, transistor gate dielectrics andcapacitor dielectrics is of fundamental importance. As device dimensionsare scaled down, thinner dielectric layers are required to maintaincapacitor and transistor performance at acceptable levels. When silicondioxide layers, which have been grown by conventional wet or dry thermaloxidation processes, are thinned, device reliability typically suffers.It is generally believed that the quality of silicon dioxide layers isinversely proportional to the number of interface states (i.e., danglingsilicon bonds) present per unit volume. In other words, quality andlong-term reliability of a silicon dioxide layer will increase as thenumber of dangling silicon bonds is reduced.

Oxidation of silicon in an ambient-containing nitrous oxide (N₂O) hasbeen reported as a means for improving the quality of silicon dioxidedielectric layers. A lower number of interface states, greaterreliability, improved hot carrier immunity and improved diffusionbarrier characteristics have been reported for silicon dioxide layersgrown in this manner as compared to those grown via conventional dry orwet oxidation processes. The increased reliability of such dielectriclayers may be due in part to the incorporation of nitrogen atoms intothe silicon dioxide matrix. The bonds between silicon atoms and nitrogenatoms are considerably stronger than those between silicon atoms andoxygen atoms. Thus, the presence of nitrogen atoms in the matrixapparently fortifies the dielectric layer against voltage-induceddegradation. However, the process of oxidizing silicon in nitrous oxidetends to be self-limiting because the silicon dioxide layer which formshas a high nitrogen content. In fact, the layer may be better identifiedas silicon oxynitride. Once a thin layer of silicon oxynitride hasformed on the surface, the diffusion of oxidant species to theunderlying silicon is greatly hampered. Thus, only very thin dielectriclayers are readily grown using this process.

Although the addition of small amounts of a strong oxidizer such as SF₆or NF₃ to a nitrous oxide ambiance has been demonstrated to increase theoxidation rate by as much as one order of magnitude, the inclusion offluorine in a silicon dioxide layer has been shown to have an adverseimpact on device reliability.

What is needed is an improved process for oxidizing silicon whichutilizes nitrous oxide as the primary oxidant species, and which doesnot rely on fluorine-containing additives to overcome the self-limitingnature of the reaction. Such a process might produce silicon dioxidelayers having both a low number of interface states and a sufficientthickness for use as transistor gate dielectrics.

BRIEF SUMMARY OF THE INVENTION

An improved process for growing high-quality silicon dioxide films orlayers on a silicon surface in a nitrous oxide-containing ambiance isdisclosed. The silicon surface is subjected to a gaseous mixture ofnitrous oxide (N₂O) and ozone (O₃). The presence of O₃ in the oxidizingambiance greatly enhances the oxidation rate compared to an ambiance inwhich N₂O is the only oxidizing agent. It is hypothesized that thepresence of O₃ promotes growth of a dielectric layer that ispredominantly silicon dioxide, although small numbers of nitrogen atomsare incorporated in the silicon dioxide matrix. Thus, a siliconoxynitride layer (i.e., a silicon dioxide layer having a high nitrogencontent) having a high degree of impermeability to the oxidant speciesis prevented from forming. The presence of O₃ in the oxidizing ambiancedoes not impair oxide reliability, as is the case when silicon isoxidized with N₂O in the presence of a strong, fluorine-containingoxidizing agent such as NF₃ or SF₆.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a flow chart which outlines the steps of a preferredembodiment of the process.

DETAILED DESCRIPTION OF THE INVENTION

This invention is disclosed in the context of a process for growinghigh-quality dielectric films or layers on the surface of asilicon-containing layer or substrate. The surface is subjected to agaseous mixture of nitrous oxide (N₂O) and ozone (O₃). The partialpressures of ozone should, preferably, be at least 10 percent of that ofnitrous oxide in order to sustain an acceptable oxidation rate. Althoughthe dielectric layers grown by this process are predominantly silicondioxide, small numbers of nitrogen atoms are incorporated in the silicondioxide matrix.

It is axiomatic that as the partial pressure of ozone with respect tothat of nitrous oxide decreases, the nitrogen content of the dielectricfilm will increase and the reaction rate will decrease. When asilicon-containing layer is oxidized in an ambiance in which N₂O is thesole oxidant, a layer of silicon oxynitride forms on the surface of thesilicon-containing layer. This silicon oxynitride layer containssufficient amounts of silicon nitride so that diffusion of N₂O to theunderlying silicon-containing layer is greatly hampered. The presence ofO₃ in the oxidizing ambiance greatly enhances the oxidation ratecompared to an ambiance in which N₂O is the only oxidizing agent. Thus,it is hypothesized that the presence of O₃ in the oxidizing ambiancedecreases the amount of nitrogen (in the form of silicon nitride) thatis incorporated in the silicon dioxide matrix. The presence of O₃ in theoxidizing ambiance does not impair oxide reliability, as is the casewhen silicon is oxidized with N₂O in the presence of a strong,fluorine-containing oxidizing agent such as NF₃ or SF₆.

Oxidation of a silicon surface in the presence of ozone and nitrousoxide may be accomplished in a variety of ways which relate to theapparatus in which the reaction is performed, the temperature at whichthe reaction is performed, the pressure at which the reaction isperformed, and the presence of other chemical species during thereaction.

The process may be performed within a sealed chamber or in an unsealedchamber such as a rapid thermal anneal chamber that is open to theatmosphere. In such an open chamber, the silicon surface is bathed inthe oxidizing ambiance. In an open chamber, the reaction proceeds at ornear atmospheric pressure. However, in a sealed chamber, the reactionmay be performed at pressures greater or less than atmospheric pressure.A range of 1 to 7,600 torr is deemed to be a useful pressure range forthe reaction.

Likewise, the process may be performed within a wide temperature range.A useful temperature range for the reaction in a manufacturingenvironment is deemed to be 600 to 1,100° C. The limit of 1,100° C. isconsidered to be the maximum desirable temperature for the processing ofintegrated circuits fabricated on a silicon substrate. Below 600° C.,the reaction rate drops off significantly.

FIG. 1 provides a flow chart that summarizes the steps of a preferredembodiment of the dielectric layer formation process. Step A indicatesthat the object to be placed in the chamber has a silicon surface.Although the surface will normally be a material that, compositionally,is relatively pure silicon, such as single crystalline orpolycrystalline silicon, the silicon may incorporate low levels ofimpurities such as dopants.

Still referring to FIG. 1, step B recites that nitrous oxide and ozonegases are introduced into the chamber. Because of reliability problemsassociated with the use of fluorine during oxidation, the mixtureshould, ideally, contain no fluorine-containing gases. However, theoxidizing ambiance, in addition to ozone and nitrous oxide, may alsocontain trace amounts of chlorine or bromine-containing gases, such asCl₂, Br₂, HCl or HBr. The environment may also contain steam. Theamounts of the halogens or halogen compounds present are chosen suchthat halogen etching is not the dominant mechanism. If employed at all,concentrations of the halogens or halogen compounds relative to theconcentration of ozone are contemplated to be in the range of no greaterthan 1:10. However, relative concentrations within a range of 1:1,000 to1:100 are deemed preferable. In another embodiment of the process, theoxidizing ambiance may also contain steam. Also contemplated areoxidizing environments containing not only ozone and nitrous oxide, butcombinations of one or more of the aforenamed halogen-containing gasesor one or more of the aforenamed halogen-containing gases and steam.Silicon wafers may be processed individually in a single waferprocessor, or multiple wafers may be processed in a conventionalvertical thermal reactor.

Still referring to FIG. 1, step C indicates that the object should bemaintained at a temperature within a range sufficient to initiate areaction between the silicon on the surface of the object and the ozoneand nitrous oxide gases. Although, at least from a theoreticalstandpoint, a certain amount of oxidation may occur at temperaturesconsiderably less than 100° C., the pace of the oxidation reaction atsuch temperatures is so slow as to be totally impractical from amanufacturing standpoint. Step D of FIG. 1 is, of course, a logicalrequirement for any manufacturing-worthy process, as film thickness mustnormally be carefully monitored.

Silicon dioxide dielectric layers grown with the new silicon oxidationprocess may be utilized in applications wherever conventionally grownsilicon dioxide layers are used. Such applications include use ascapacitor dielectrics, use in field oxide layers grown via conventionallocal oxidation of silicon (LOCOS) processes or via modified LOCOSprocesses, and use as both floating gate and control gate dielectriclayers for field effect transistors.

Although only several embodiments of the invention have been disclosedherein, it will be obvious to those having ordinary skill in the art ofintegrated circuit manufacture that modifications and changes may bemade thereto without departing from the scope and the spirit of theinvention as hereinafter claimed.

1. A process for forming a dielectric film on an exposed surface of alayer containing silicon in a chamber, the process comprising:determining a desired thickness of the dielectric film, the dielectricfilm including silicon dioxide and silicon nitride therein when formedon the exposed surface of the layer containing silicon when the layer ismaintained at a temperature in the range of at least 600° C. to 1100° C.in an oxidizing atmosphere; introducing into the chamber a gaseousmixture including nitrous oxide exhibiting a partial pressure, ozoneexhibiting a partial pressure, at least one compound containing ahalogen selected from the group consisting of Cl₂, Br₂, HCl and HBr, andsteam, the partial pressure of the ozone being at least one tenth thepartial pressure of the nitrous oxide in the gaseous mixture, thegaseous mixture being substantially free of fluorine-containing gases;and forming the dielectric film of the desired thickness by contactingthe exposed surface of the layer containing silicon with the gaseousmixture including nitrous oxide, ozone, at least one compound containinga halogen selected from the group consisting of Cl₂, Br₂, HCl and HBr,and steam.
 2. The process of claim 1, wherein the chamber is sealed. 3.The process of claim 2, wherein the chamber is maintained at a pressurewithin a range of 1 to 7,600 torr.
 4. A process for forming afield-effect transistor gate dielectric layer on an exposed surface of alayer of polycrystalline silicon having a desired thickness in achamber, the process comprising: determining the desired thickness ofthe field-effect transistor gate dielectric layer, the field-effecttransistor gate dielectric layer containing silicon dioxide and siliconnitride when formed on the exposed surface of the layer ofpolycrystalline silicon when the layer is maintained at a temperature inthe range of at least 600° C. to 1100° C. in an oxidizing atmosphere;providing a gaseous mixture including nitrous oxide, ozone, at least onecompound containing a halogen selected from the group consisting of Cl₂,Br₂, HCl and HBr, and steam, the gaseous mixture being substantiallyfree of fluorine-containing gases; and forming the field-effecttransistor gate dielectric layer to the desired thickness by subjectingthe exposed surface of the layer of polycrystalline silicon to thegaseous mixture, the gaseous mixture including the nitrous oxide at apartial pressure and the ozone at a partial pressure, the partialpressure of the ozone being at least one tenth the partial pressure ofthe nitrous oxide.
 5. The process of claim 4, wherein the chamber issealed.
 6. The process of claim 4, wherein the chamber is maintained ata pressure within a range of 1 to 7,600 torr.